The present application relates generally to the field of capacitance sensing measurements used for classifying an occupant in a seat of a vehicle and, more particularly, an apparatus and method for eliminating cross talk between the wires connecting the capacitance sensor and other wires in an occupant classification system.
In order to protect vehicle occupants during vehicle crashes, a vehicle may contain one or more safety device that are activated during a vehicle crash. These safety devices are well known in the art and include air bags and seat belt pretensioners. It is usually desirable to configure an automatic restraint system to activate the safety devices to protect the vehicle occupant during the crash event while, at the same time, preventing the safety devices themselves from injuring the occupant. For example, one situation may involve the use of a seat belt pretensioner and an air bag to protect a vehicle occupant in the case of a crash event. If the vehicle occupant is an adult, it may be desirable to have the seat belt pretensioner and the air bag be utilized during the crash event so as to protect the adult occupant from impacting an interior structure of the vehicle. If the vehicle occupant is a child, the impact from a deployed airbag may be injurious to the child. Thus, it may be desirable to disable the deployment of the air bag during the crash event if it is determined that the vehicle occupant is a child. Thus, the classification of whether a particular occupant is a child or an adult should be ascertained.
Besides mitigating any injury to the vehicle occupant, occupant classification is also desirable to prevent the unnecessary use of a particular safety device. For example, one situation may involve the use of two safety devices in which only one safety device is necessary when the vehicle occupant is a child and two safety devices are necessary when the vehicle occupant is an adult. The reason is that an adult usually has a greater mass and, as a result, greater momentum in which to hit an internal structure of the vehicle during a crash event. Conversely, a child has less mass and will have a smaller momentum during a crash event. As a result, the adult may have a greater need of the second safety device so as to absorb the additional momentum caused by the adult's greater mass. A child will not need the second safety device, and it is desirable to prevent the unnecessary use of the additional safety device in such a circumstance so as to avoid the time and expense of replacing or resetting the second safety device. Thus, the classification of whether a particular occupant is a child or an adult should be ascertained for this reason as well.
One approach for detecting whether a particular occupant in a particular seat is an adult or child is by sensing the influence of the occupant upon an electric field generated proximate to the seat on which the occupant is situated. A capacitance sensor may detect a change in the electric field caused by the occupant because the occupant has dielectric and conductive properties that can influence an electric field.
A capacitance sensor measures the capacitance of one or more electrodes using the relationship between received and applied signals for a given electrode configuration. U.S. Patent Application Publication 2004/0113634 (U.S. patent application Ser. No. 10/707,238, filed on Nov. 30, 2003) (herein incorporated by reference in its entirety) discloses an occupant sensor used to distinguish a child or infant from an adult. For example, a normally seated adult occupant provides a substantially larger increase in capacitance relative to an empty seat, than does a child in a rear facing child seat. The occupant sensor can discriminate a child seat from an adult because the child in a child seat does not have a large surface of its body very near to the sensor electrode contained in the seat bottom. Usually there is a significant gap, up to several inches, between the child in the child seat and the seat bottom. Because child seats are typically made of plastic, the seats themselves are not sensed directly by the occupant sensor. Because only a small portion of the surface of the child is near to the sensor electrode, the capacitance measured by the capacitance sensor is relatively low, and more particularly, less than the threshold capacitance for detecting a normally seated adult occupant.
The capacitance sensor are usually disposed on sensing pads connected to a vehicle seat. Wires in a sensing harness connect the capacitance sensor to an electronic control unit (ECU), which detects signals from the capacitance sensors and determines whether the vehicle occupant is a child or an adult. However, if another wire that is not connected to the capacitance sensor comes from the same connector and is grounded (for example, inside the ECU), the distance between the capacitance sensor wires and the grounded wire will significantly affect the capacitive coupling to ground. This effect is known as cross talk. Cross talk may be caused by any wire not connected to the sensor electrodes in the sensor harness connecting the ECU to the sensing pad. Because the simplest harness design is a group of discrete wires next to each other, the spacing and orientation of the wires relative to each other is not known or held constant. Thus, changes in the spacing and orientation of the wires could change the capacitive current out to the sensing pad and provide a false reading, such as making an empty seat appear to be a seat with an adult in it, a seated adult may appear to be an empty seat, a seated child may appear to be a seated adult, or a seated adult may appear to be a seated child. Cross talk can also be caused by components not contained in the sensing harness. For example, if the other elements on the sensing pad shift relative to each other, the current sent out to the sensing element could also change.
Currently, one technique to reduce or eliminate cross talk is to use special connectors and coaxial cable to separate the capacitance sensor wires and other wires in the system. Such a technique may be costly. Thus, there is a need for a capacitance sensing apparatus for use in an occupant classification system that can reduce or eliminate cross talk which does not rely on special connectors or coaxial cable.